1. Field of the Invention
The present invention relates to an echo canceller of an ADSL system, and more particularly, to a method of training the echo canceller of the ADSL system.
2. Description of the Related Art
Generally, an echo canceller is a device for canceling a received signal reflected through a hybrid and/or a twisted-pair cable connected to other units in a communication system.
The contemporary asymmetric high speed digital subscriber line (ADSL) communication system partitions signals into a transmitted signal and a received signal by using an analog hybrid. The signals are properly and completely divided only when impedance is matched exactly. For example, if a communication channel is changed, or if passive elements of an analog hybrid have errors, the impedance is not exactly matched, and as a consequence, a transmitted signal of a transmitting system is input to a receiver of the transmitting system.
Echo cancellers include the finite impulse response (FIR) echo canceller, the infinite impulse response (IIR) echo canceller and the interpolated FIR echo canceller. For effective operation, the echo canceller should be able to monitor and learn the echo channels of a transmission medium to the highest degree possible and to remove the echo of a transmitted signal at a receiver irrespective of the echo canceller configuration.
An FIR echo canceller is generally the simplest and most adaptable form of echo canceller. The coefficient of the echo canceller is set so that the echo canceller can imitate echo channels. In general, the FIR echo canceller is tuned so that a squared error between the echo channel and the echo canceller is at a minimum. However, while the FIR echo canceller is used to imitate a general echo channel, the number of taps of the echo canceller can vary. Accordingly, the degree of calculation and complexity is increased.
The IIR echo canceller offers the advantages of a relatively small amount of calculation requirements and a relatively simple hardware configuration; however, stability of the IIR echo canceller is somewhat limited according to pole or poles of the IIR echo canceller. There are many problems in a digital echo canceller associated with training an echo channel. If the pole(s) and the zero(s) of the IIR echo canceller are not known, some problems can occur in the IIR echo canceller since the pole(s) and the zero(s) of the IIR echo canceller cannot be determined exactly. In this case, the coefficient of the IIR echo canceller is usually found using adaptive signal processing techniques. However, in this case, attention should be always paid to the stability of the IIR echo canceller.
The interpolated FIR echo canceller includes a number of stages of FIR echo cancellers so that the performance of the interpolated FIR echo canceller is better than that of the single-stage FIR echo canceller. However, the interpolated FIR echo canceller is more complex in its hardware and requires additional calculations in determining, or learning, the characteristics of an echo channel.